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  • richardmitnick 7:45 pm on April 28, 2016 Permalink | Reply
    Tags: , Grand Unified Theory,   

    From Symmetry: “A GUT feeling about physics” 

    Symmetry Mag

    Symmetry

    04/28/16
    Matthew R. Francis

    Scientists want to connect the fundamental forces of nature in one Grand Unified Theory.

    1
    Artwork by Sandbox Studio, Chicago

    The 1970s were a heady time in particle physics. New accelerators in the United States and Europe turned up unexpected particles that theorists tried to explain, and theorists in turn predicted new particles for experiments to hunt. The result was the Standard Model of particles and interactions, a theory that is essentially a catalog of the fundamental bits of matter and the forces governing them.

    While that Standard Model is a very good description of the subatomic world, some important aspects—such as particle masses—come out of experiments rather than theory.

    The Standard Model of elementary particles , with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.
    The Standard Model of elementary particles , with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    “If you write down the Standard Model, quite frankly it’s a mess,” says John Ellis, a particle physicist at King’s College London. “You’ve got a whole bunch of parameters, and they all look arbitrary. You can’t convince me that’s the final theory!”

    The hunt was on to create a grand unified theory, or GUT, that would elegantly explain how the universe works by linking three of the four known forces together. Physicists first linked the electromagnetic force, which dictates the structure of atoms and the behavior of light, and the weak nuclear force, which underlies how particles decay.

    But they didn’t want to stop there. Scientists began working to link this electroweak theory with the strong force, which binds quarks together into things like the protons and neutrons in our atoms. (The fourth force that we know, gravity, doesn’t have a complete working quantum theory, so it’s relegated to the realm of Theories of Everything, or ToEs.)

    Linking the different forces into a single theory isn’t easy, since each behaves a different way. Electromagnetism is long-ranged, the weak force is short-ranged, and the strong force is weak in high-energy environments such as the early universe and strong where energy is low. To unify these three forces, scientists have to explain how they can be aspects of a single thing and yet manifest in radically different ways in the real world.

    The electroweak theory unified the electromagnetic and weak forces by proposing they were aspects of a single interaction that is present only at very high energies, as in a particle accelerator or the very early universe. Above a certain threshold known as the electroweak scale, there is no difference between the two forces, but that unity is broken when the energy drops below a certain point.

    The GUTs developed in the mid-1970s to incorporate the strong force predicted new particles, just as the electroweak theory had before. In fact, the very first GUT showed a relationship between particle masses that allowed physicists to make predictions about the second-heaviest particle before it was detected experimentally.

    “We calculated the mass of the bottom quark before it was discovered,” says Mary Gaillard, a particle physicist at University of California, Berkeley. Scientists at Fermilab would go on to find the particle in 1977.

    GUTs also predicted that protons should decay into lighter particles. There was just one problem: Experiments didn’t see that decay.

    The problem with protons

    GUTs predicted that all quarks could potentially change into lighter particles, including the quarks making up protons. In fact, GUTs said that protons would be unstable over a period much longer than the lifetime of the universe. To maximize the chances of seeing that rare proton decay, physicists needed to build detectors with a lot of atoms.

    However, the first Kamiokande experiment in Japan didn’t detect any proton decays, which meant a proton lifetime longer than that predicted by the simplest GUT theory. More complicated GUTs emerged with longer predicted proton lifetimes – and more complicated interactions and additional particles.

    Super-Kamiokande Detector
    Super-Kamiokande Detector

    More complicated GUTs emerged with longer predicted proton lifetimes – and more complicated interactions and additional particles.

    Most modern GUTs mix in supersymmetry (SUSY), a way of thinking about the structure of space-time that has profound implications for particle physics. SUSY uses extra interactions to adjust the strength of the three forces in the Standard Model so that they meet at a very high energy known as the GUT scale.

    Standard model of Supersymmetry Illustration: CERN & IES de SAR
    Standard model of Supersymmetry Illustration: CERN & IES de SAR

    “Supersymmetry gives more particles that are involved via virtual quantum effects in the decay of the proton,” says JoAnne Hewett, a physicist at the Department of Energy’s SLAC National Accelerator Laboratory. That extends the predicted lifetime of the proton beyond what previous experiments were able to test. Yet SUSY-based GUTs also have some problems.

    “They’re kinda messy,” Gaillard says. Particularly, these theories predict more Higgs-like particles and different ways the Higgs boson from the Standard Model should behave. For that reason, Gaillard and other physicists are less enamored of GUTs than they were in the 1970s and ’80s. To make matters worse, no supersymmetric particles have been found yet. But the hunt is still on.

    “The basic philosophical impulse for grand unification is still there, just as important as ever,” Ellis says. “I still love SUSY, and I also am enamored of GUTs.”

    Hewett agrees that GUTs aren’t dead yet.

    “I firmly believe that an observation of proton decay would affect how every person would think about the world,” she says. “Everybody can understand that we’re made out of protons and ‘Oh wow! They decay.’”

    Upcoming experiments like the proposed Hyper-K in Japan and the Deep Underground Neutrino Experiment in the United States will probe proton decay to greater precision than ever.

    Hyper-Kamiokande
    Hyper-Kamiokande

    FNAL LBNF/DUNE
    FNAL LBNF/DUNE

    Seeing a proton decay will tell us something about the unification of the forces of nature and whether we ultimately can trust our GUTs.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 10:12 am on October 1, 2015 Permalink | Reply
    Tags: , Grand Unified Theory, ,   

    From Lawrence Krauss at Nautilus: “The Trouble with Theories of Everything” 

    Nautilus

    Nautilus

    October 1, 2015

    Lawrence Krauss
    By Lawrence M. Krauss
    Illustrations by Melinda Beck

    Whenever you say anything about your daily life, a scale is implied. Try it out. “I’m too busy” only works for an assumed time scale: today, for example, or this week. Not this century or this nanosecond. “Taxes are onerous” only makes sense for a certain income range. And so on.

    Surely the same restriction doesn’t hold true in science, you might say. After all, for centuries after the introduction of the scientific method, conventional wisdom held that there were theories that were absolutely true for all scales, even if we could never be empirically certain of this in advance. [Sir Isaac] Newton’s universal law of gravity, for example, was, after all, universal! It applied to falling apples and falling planets alike, and accounted for every significant observation made under the sun, and over it as well.

    With the advent of relativity, and general relativity in particular, it became clear that Newton’s law of gravity was merely an approximation of a more fundamental theory. But the more fundamental theory, general relativity, was so mathematically beautiful that it seemed reasonable to assume that it codified perfectly and completely the behavior of space and time in the presence of mass and energy.

    The advent of quantum mechanics changed everything. When quantum mechanics is combined with relativity, it turns out, rather unexpectedly in fact, that the detailed nature of the physical laws that govern matter and energy actually depend on the physical scale at which you measure them. This led to perhaps the biggest unsung scientific revolution in the 20th century: We know of no theory that both makes contact with the empirical world, and is absolutely and always true. (I don’t envisage this changing anytime soon, string theorists’ hopes notwithstanding.) Despite this, theoretical physicists have devoted considerable energy to chasing exactly this kind of theory. So, what is going on? Is a universal theory a legitimate goal, or will scientific truth always be scale-dependent?

    1

    The combination of quantum mechanics and relativity implies an immediate scaling problem. Heisenberg’s famous uncertainty principle, which lies at the heart of quantum mechanics, implies that on small scales, for short times, it is impossible to completely constrain the behavior of elementary particles. There is an inherent uncertainty in energy and momenta that can never be reduced. When this fact is combined with special relativity, the conclusion is that you cannot actually even constrain the number of particles present in a small volume for short times. So called “virtual particles” can pop in and out of the vacuum on timescales so short you cannot measure their presence directly.

    One striking effect of this is that when we measure the force between electrons, say, the actual measured charge on the electron—the thing that determines how strong the electric force is—depends on what scale you measure it at. The closer you get to the electron, the more deeply you are penetrating inside of the “cloud” of virtual particles that are surrounding the electron. Since positive virtual particles are attracted to the electron, the deeper you penetrate into the cloud, the less of the positive cloud and more of the negative charge on the electron you see.

    Then, when you set out to calculate the force between two particles, you need to include the effects of all possible virtual particles that could pop out of empty space during the period of measuring the force. This includes particles with arbitrarily large amounts of mass and energy, appearing for arbitrarily small amounts of time. When you include such effects, the calculated force is infinite.

    Richard Feynman shared the Nobel Prize for arriving at a method to consistently calculate a finite residual force after extracting a variety of otherwise ambiguous infinities. As a result, we can now compute, from fundamental principles, quantities such as the magnetic moment of the electron to 10 significant figures, comparing it with experiments at a level unachievable in any other area of science.

    But Feynman was ultimately disappointed with what he had accomplished—something that is clear from his 1965 Nobel lecture, where he said, “I think that the renormalization theory is simply a way to sweep the difficulties of the divergences of electrodynamics under the rug.” He thought that no sensible complete theory should produce infinities in the first place, and that the mathematical tricks he and others had developed were ultimately a kind of kludge.

    Now, though, we understand things differently. Feynman’s concerns 
were, in a sense, misplaced. The problem was not with the theory, but with trying to push the theory beyond the scales where it provides the correct description of nature.

    There is a reason that the infinities produced by virtual particles with arbitrarily
 large masses and energies are not physically relevant: They are
 based on the erroneous 
presumption that the
 theory is complete. Or,
 put another way, that the theory describes physics on all scales, even arbitrarily small scales of distance and time. But if we expect our theories to be complete, that means that before we can have a theory of anything, we would first have to have a theory of everything—a theory that included the effects of all elementary particles we already have discovered, plus all the particles we haven’t yet discovered! That is impractical at best, and impossible at worst.

    Thus, theories that make sense must be insensitive, at the scales we can measure in the laboratory, to the effects of possible new physics at much smaller distance scales (or less likely, on much bigger scales). This is not just a practical workaround of a temporary problem, which we expect will go away as we move toward ever-better descriptions of nature. Since our empirical knowledge is likely to always be partially incomplete, the theories that work to explain that part of the universe we can probe will, by practical necessity, be insensitive to possible new physics at scales beyond our current reach. It is a feature of our epistemology, and something we did not fully appreciate before we began to explore the extreme scales where quantum mechanics and relativity both become important.

    This applies even to the best physical theory we have in nature: quantum electrodynamics, which describes the quantum interactions between electrons and light. The reason we can, following Feynman’s lead, throw away with impunity the infinities that theory produces is that they are artificial. They correspond to extrapolating the theory to domains where it is probably no longer valid. Feynman was wrong to have been disappointed with his own success in maneuvering around these infinities—that is the best he could have done without understanding new physics at scales far smaller than could have been probed at the time. Even today, half a century later, the theory that takes over at the scales where quantum electrodynamics is no longer the correct description is itself expected to break down at still smaller scales.

    2

    There is an alternative narrative to the story of scale in physical theory. Rather than legitimately separating theories into their individual domains, outside of which they are ineffective, scaling arguments have revealed hidden connections between theories, and pointed the way to new unified theories that encompass the original theories and themselves apply at a broader range of scale.

    For example, all of the hoopla over the past several years associated with the discovery of the Higgs particle was due to the fact that it was the last missing link in a theory that unifies quantum electrodynamics with another force, called the weak interaction. These are two of the four known forces in nature, and on the surface they appear very different. But we now understand that on very small scales, and very high energies, the two forces can be understood as different manifestations of the same underlying force, called the electroweak force.

    Scale has also motivated physicists to try
 to unify another of 
nature’s basic forces,
 the strong force, into
 a broader theory. The 
strong force, which 
acts on the quarks 
that make up protons
 and neutrons, resisted
 understanding until
 1973. That year, three
 theorists, David Gross,
 Frank Wilczek, and 
David Politzer, demonstrated something 
absolutely unexpected 
and remarkable. They
 demonstrated that a 
candidate theory to
 describe this force, called quantum chromodynamics—in analogy with quantum electrodynamics—possessed a property they called Asymptotic Freedom.

    Asymptotic Freedom causes the strong force between quarks to get weaker as the quarks are brought closer together. This explained not only an experimental phenomenon that had become known as “scaling”—where quarks within protons appeared to behave as if they were independent non-interacting particles at high energies and small distances—but it also offered the possibility to explain why no free quarks are observed in nature. If the strong force becomes weaker at small distances, it presumably can be strong enough at large distances to ensure that no free quarks ever escape their partners.

    The discovery that the strong force gets weaker at small distances, while electromagnetism, which gets united with the weak force, gets stronger at small distances, led theorists in the 1970s to propose that at sufficiently small scales, perhaps 15 orders of magnitude smaller than the size of a proton, all three forces (strong, weak, and electromagnetic) get unified together as a single force in what has become known as a Grand Unified Theory. Over the past 40 years we have been searching for direct evidence of this—in fact the Large Hadron Collider is just now searching for a whole set of new elementary particles that appear to be necessary for the scaling of the three forces to be just right.

    CERN LHC Map
    CERN LHC Grand Tunnel
    CERN LHC particles
    LHC at CERN

    But while there is indirect evidence, no direct smoking gun has yet been found.

    Naturally, efforts to unify three of the four known forces led to further efforts to incorporate the fourth force, gravity, into the mix. In order to do this, proposals have been made that gravity itself is merely an effective theory and at sufficiently small scales it gets merged with the other forces, but only if there are a host of extra spatial dimensions in nature that we do not observe. This theory, often called superstring theory, produced a great deal of excitement among theorists in the 1980s and 1990s, but to date there is not any evidence that it actually describes the universe we live in.

    If it does then it will possess a unique and new feature. Superstring theory may ultimately produce no infinities at all. Therefore, it has the potential to apply at all distance scales, no matter how small. For this reason it has become known to some as a “theory of everything”—though, in fact, the scale where all the exotica of the theory would actually appear is so small as to be essentially physically irrelevant as far as foreseeable experimental measurements would be concerned.

    The recognition of the scale dependence of our understanding of physical reality has led us, over time, toward a proposed theory—string theory—for which this limitation vanishes. Is that effort the reflection of a misplaced audacity by theoretical physicists accustomed to success after success in understanding reality at ever-smaller scales?

    While we don’t know the answers to that question, we should, at the very least, be skeptical. There is no example so far where an extrapolation as grand as that associated with string theory, not grounded by direct experimental or observational results, has provided a successful model of nature. In addition, the more we learn about string theory, the more complicated it appears to be, and many early expectations about its universalism may have been optimistic.

    At least as likely is the possibility that nature, as Feynman once speculated, could be like an onion, with a huge number of layers. As we peel back each layer we may find that our beautiful existing theories get subsumed in a new and larger framework. So there would always be new physics to discover, and there would never be a final, universal theory that applies for all scales of space and time, without modification.

    Which road is the real road to reality is up for grabs. If we knew the correct path to discovery, it wouldn’t be discovery. Perhaps my own predilection is just based on a misplaced hope of continued job security for physicists! But I also like the possibility that there will forever be mysteries to solve. Because life without mystery can get very boring, at any scale.

    Lawrence M. Krauss is a theoretical physicist and cosmologist, Director of the Origins Project and Foundation Professor in
 the School of Earth and Space Exploration at Arizona State University. He is also the author of bestselling books including A Universe from Nothing and The Physics of Star Trek.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    Welcome to Nautilus. We are delighted you joined us. We are here to tell you about science and its endless connections to our lives. Each month we choose a single topic. And each Thursday we publish a new chapter on that topic online. Each issue combines the sciences, culture and philosophy into a single story told by the world’s leading thinkers and writers. We follow the story wherever it leads us. Read our essays, investigative reports, and blogs. Fiction, too. Take in our games, videos, and graphic stories. Stop in for a minute, or an hour. Nautilus lets science spill over its usual borders. We are science, connected.

     
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